Warren Pennick of Lansdale pleaded guilty but mentally ill to third degree murder in Montgomery County Court Tuesday.

Assistant District Attorney Jordan Friter reported 55-year-old Pennick was sentenced to 20-40 years in prison for the Halloween 2012 slaying of his mother.

Investigators said Pennick repeatedly stabbed his 79-year-old mother, Lorraine Pennick, and slashed her throat in their Lansdale home before leaving the 7-inch knife on her chest, changing clothes and walking to the police station to confess.